1. Field of the Invention
The present invention relates to a method and system for image reconstruction in cone-beam X-ray computed tomography and, in particular, to a method and system using mapping coefficients to reconstruct the image.
2. Discussion of the Background
Cone beam computed tomography (CT) reconstructs the interior of an object or It( patient from the two-dimensional projections of transmitted x-rays through the object or patient. Feldkamp, Kress, and Davis introduced an approximate but computationally efficient image reconstruction algorithm, termed convolution backprojection, for the case of a circular orbit of the X-ray source around an object or patient. The algorithm assumes that the rotational axis is fixed and that the mid-plane of the x-rays is fixed.
In recent years, there has been an attempt to implement cone-beam CT on C-arm gantries that only revolve around a portion of the object or patient during imaging. In medical CT, C-arm gantries are desirable because they provide some access to a patient for simultaneous intervention by medical personnel. Furthermore, many patients prefer not to be entirely surrounded by the CT instrument. C-arm gantries are also capable of imaging a portion of a large object that otherwise would not fit within a traditional CT instrument,
The use of C-arm gantries for CT has, however, been limited. Most C-arm gantries xe2x80x9cwobblexe2x80x9d as they rotate, thus violating the underlying assumptions of the Feldkamp, Kress, and Davis algorithm. The term wobble is used to denote any of a wide range of non-idealities resulting in a non-circular or irregular orbit or orbit portion; including but not limited to: vibration, gravitational sag, mechanical backlash, and other irregularities. The resulting non-circular orbit or orbit portion thus introduces errors into the reconstruction that limit resolution of features inside the object or patient. Nevertheless, Feldkamp backprojection is commonly used to reconstruct images in C-arm gantry systems.
Traditional calibration methods developed with the underlying assumptions of a fixed rotational axis and mid-plane of the x-rays have not begun to address the difficulties of non-circular orbits or orbit portions. For example, Picard et al. (U.S. Pat. No. 5,442,674) teaches the use of a 3D calibration phantom, preferably consisting of a plethora of cellular structures that minimally attenuate X-rays arranged in helix, to calibrate an X-ray system. Rather than addressing non-circular orbits or orbit portions around a patient or object, their method seeks xe2x80x9cthe intrinsic parameters of the systemxe2x80x9d when it is xe2x80x9cassumed that the imaging system undergoes perfect circular rotation or almost perfect circular rotation.xe2x80x9d
Others have tried to construct methods which account for non-circular orbits or orbit portions. Fahrig et al. (Med. Phys. 27(1) 30-38) briefly discuss several possible solutions, including reinforcement of the mechanical system to reduce or remove wobble and monitoring the position of the mechanical system during motion through an external system to correct for wobble post-acquisition. Their own approach is discussed in detail and includes the generation of a trigonometric expression describing the position of the gantry. This approach assumes that 1) the motion of the gantry exhibits long term reproducibility, 2) the deviations from a perfect trajectory are small, and 3) the non-idealities due to wobble are either parallel to the axis of rotation or tangential to the circle of rotation and perpendicular to the line joining the x-ray source and the detector plane.
As another example of attempts to correct for wobble in non-circular orbits or orbit portions, Wiesent et al. (U.S. Pat. No. 5,706,324) teach the simultaneous imaging of a patient and a calibration phantom with X-ray computed tomography. While it is stated the annular calibration phantom xe2x80x9cpermits a precise determination of the photographic geometry,xe2x80x9d there is no discussion about how this is accomplished.
Accordingly, one object of this invention is to provide a method and system for the reconstruction of an image through mapping.
Another object is to provide a method and system that allow for the reconstruction of an image formed by an X-ray source and detector that have a non-circular orbit or orbit portions about the patient or object.
A further object of this invention is to provide a method and a device that allow for the reconstruction from projections from any non-circular orbit or orbit portion.
These and other objects are achieved by a method for reconstructing an image of a subject including steps of exposing said subject to x-rays from a source rotated about the subject in an irregular path, obtaining exposure data, and reconstructing the image from the exposure data by mapping a reconstructed image point from a known coordinate. The method may also include a step of exposing the subject using a source mounted on an open C-arm gantry circularly rotated about the subject an irregular path containing non-idealities comprising at least one of wobble of the C-arm, gravitational sag, and vibration, or include steps of generating calibration factors from known coordinates and using the calibration factors in the reconstructing step.
The objects of the invention may also be obtained by an image reconstruction system, having an x-ray source, an x-ray detector disposed to face said source, and means for reconstructing the image from exposure data obtained from the detector by mapping a reconstructed image point from a known coordinate connected to the detector. The source and detector may be mounted on an open C-arm gantry circularly rotated about the subject an irregular path containing non-idealities comprising at least one of wobble of said C-arm, gravitational sag, and vibration.